Amnesty International has called on Egypt's military authorities to drop charges against an activist accused of defaming the country's ruling military generals on Twitter, in what could prove to be a landmark case for freedom of expression in post-Mubarak Egypt.

Asmaa Mahfouz, the co-founder of the influential 6 April movement, was summoned to a military court on Sunday after posting two sentences on the internet. "If the justice system does not give us our rights, nobody should be upset if armed groups emerge and carry out assassinations," she wrote. "As long as there is no law there is no justice, anything can happen and nobody should be upset."

Revolutionaries in Egypt have been campaigning against the use of military trials ever since the fall of former president Hosni Mubarak in February, and the issue has become a major faultline in the runup to November's parliamentary elections, when the army has promised to return to its barracks. Many have complained that the response of the Supreme Council of Armed Forces [SCAF] to legitimate criticism echoes that of the Mubarak regime.

Mahfouz, 26, who also criticised the council last month in an interview with al-Jazeera, has been released on a £2,000 bail. On Tuesday she was referred to a military tribunal, though no date has been set for her case.

General Adel Morsy, who presides over the military justice commission, defended the army's right to charge Mahfouz. "No citizen shall be [condemned] for an opinion they freely issue, as long as that opinion is objective and does not infringe on other people's rights, or defame them and their dignity," he said in a statement. "If the said opinion goes beyond those limits, punishment becomes necessary as it is considered a crime of insult."

Heba Morayef, an Egypt researcher with Human Rights Watch, said that the case highlighted the council's deliberate intimidation of critics. "The charges against Mahfouz all exist in the Egyptian penal code, even though insulting public officials – including the use of strong words – is allowed under international human rights," she said. "This highlights the need to review all repressive laws used by the government to stifle freedoms of speech, association, and assembly – all of which are under attack."

Activist Noor Ayman Noor said 10,000 military trials had taken place in recent months, including many involving peaceful protesters. "The SCAF has noticed that the public haven't taken a strong stance against these trials, so they are now expanding them to activists. Only today, I was informed that three more activists were summoned to a military hearing".